The ultimate aims of this research program are to understand in molecular detail the mechanism of DNA replication in a model eukaryote, Drosophila melanogaster, and the mechanism of homologous recombination in a prokaryote, Escherichia coli. The investigation will be organized along the following lines: 1. Mechanism of DNA replication in embryos of Drosophila melanogaster a. DNA chain elongation, i.e., initiation and elongation of Okazaki fragments (1) Assembly of a DNA polymerase-primase holoenzyme (a) Isolation of DNA polymerase core enzyme (b) Isolation of DNA polymerase-primase accessory proteins (2) Direct isolation of a DNA polymerase-primase holoenzyme b. Initiation of DNA synthesis at a replication origin c. Regulation of DNA replication (1) Investigation of factors that control rate of DNA replication (2) Identification of DNA polymerase-primase mutants 2. Mechanism of DNA strand exchange promoted by recA protein of Escherichia coli a. Formation of non-dissociable recA protein-single-stranded DNA complex b. Mechanism of initial pairing between single-stranded and duplex DNAs (1) Processive versus distributive search for homology (2) Initiation of strand assimilation at a nick in duplex DNA (3) Strand assimilation in the presence of nonhydrolyzable ATP analogues c. Role of ATP hydrolysis in the branch migration phase of DNA strand exchange (1) Search for factors that increase efficiency of branch migration (2) Search for factors that increase rate of branch migration d. Recycling of recA protein following strand exchange e. Mechanism of recA protein-catalyzed annealing of complementary single strands.